The present invention relates to a pneumatic tire having projections standing out from the bottom of grooves in the tire tread to prevent stones from being trapped therein.
In vehicles which travel on dirt roads such as trucks, construction vehicles and the like, small foreign objects such as stones, pebbles and the like are liable to be trapped in the tread grooves on the tires of the vehicles.
While travelling, the trapped stones are repeatedly pressed against the groove walls and groove bottom. As a result, the groove bottom and the structure inside the tire (such as reinforcing cord layers) are often damaged seriously. Therefore, not only the durability of the tread portion but also the retread rate of the used tires decreases.
In order to avoid stone trapping, many ideas have been proposed. For example, in the laid-open Japanese patent application Nos. JP-A-60-189608 and JP-A-61-291203, a tire having a rib type tread pattern is disclosed, wherein a circumferentially and continuously extending zigzag groove is provided therein with circumferentially discontinuous projections. In U.S. Pat. No. 5160385, a tire having of a block type tread pattern is disclosed, wherein circumferentially continuously extending grooves are provided therein with circumferentially discontinuous projections. The circumferential grooves are interconnected by axial grooves, and some of the projections disposed at the junctions of the circumferential grooves and axial grooves have branches which extend into the axial grooves but immediately end.
In general, stone trapping is liable to occur in a central tread region where ground pressure is relatively high. If a block pattern tire is made according to the above-mentioned assumptions, then in the center tread center region, stones are still liable to be trapped in the circumferential groove between the projections and also in the axial grooves. Thus, the tire may be damaged.